PLATELET-DERIVED GROWTH-FACTOR CAUSES SUSTAINED DEPLETION OF BOTH INOSITOL TRISPHOSPHATE-SENSITIVE AND CAFFEINE-SENSITIVE INTRACELLULAR CALCIUM STORES IN VASCULAR SMOOTH-MUSCLE CELLS

Since the platelet-derived growth factor (PDGF)induced increase in cellular inositol 1,4,5-trisphosphate (InsP(3)) has been found to decay to basal levels soon after the onset of PDGF exposure, it has been argued that activation of Ca2+ release from intracellular stores must be similarly transient. The possibility remains, however, that PDGF-induced release of stored Ca2+ is initiated and sustained by other second-messenger systems. To test the hypothesis that PDGF-BB initiates sustained Ca2+ release from cellular stores, we performed 4-hour Ca-45 effluxes on monolayers of A7r5 vascular smooth muscle cells in small, continuously perfused chambers. Isoform PDGF-BB (5 ng/mL for 30 minutes or 30 ngimL for 15 minutes) was added to the perfusate beginning at 30 minutes of efflux. A dose-related increase in Ca-45 release was sustained as long as PDGF-BB was present. Detailed kinetic analysis and nonlinear least-squares fitting of the experimental data revealed that (1) PDGF-BB induced sustained increases of 2.86-fold (5 ng/mL) and 6.50-fold (30 ng/mL) in the rate constant governing Ca2+ release from intracellular stores, (2) the apparent K-m for this effect was 13.4+/-1.31 ng PDGF-BB/ mL, and (3) the entire agonist-releasable Ca2+ store (presumably sarcoplasmic reticulum) is sensitive to PDGF-BB. These data indicate that PDGF-BB causes a sustained depletion of intracellular Ca2+ stores by means of sustained activation of Ca2+ release and suggest that intraorganellar Ca2+ may be one of the signals that mediates long-term smooth muscle responses to PDGF.